In the active matrix display device such as the TFT liquid crystal display and the like, with respect to an LCD driver (drive circuit) particularly for a mobile use such as a portable telephone and the like, an LCD driver containing a frame memory is in widespread use, which aims at low power consumption through elimination of data communications between a CPU (MPU) and the LCD driver at the time of displaying a still image or a standby screen.
For the still image, the standby screen, and the like, since one frame data can be carried by the frame memory of the LCD driver, the same data from the frame memory is read, so that display can be made without performing data communications between the CPU and the LCD driver.
However, when the frame memory is carried by the LCD driver, a chip size becomes large, and the cost thereof increases. On the other hand, the demand for a low cost system is generally advancing in the market, and chip size saving of the LCD driver is solicited. Hence, the LCD driver having no cost increase or a slight cost increase when carrying the frame memory is demanded in the market.
Hence, in the LCD driver of the conventional art, a display data is RGB-YUV converted (that is, converted from an RGB format to a YUV format), and particularly, by storing the display data in the memory (frame memory) by a YUV 422 format, in other words, by reducing the amount of the memory-stored data by compression, the chip size saving is realized. Note that, Y indicates luminance, and U and V indicate chrominance.
In the following, for comparison with the present invention, the system of a display drive (system and method) in the conventional liquid crystal display device (LCD) will be briefly described below. In the conventional method, in the liquid crystal display device, before storing the display data in the memory, the inputted display RGB data of 24 bits are serial/parallel converted, and the data of RGB 24 bits×2 for the parallelized horizontal two pixels portion is converted by YUV data, and is converted into a YUV 422 format.
Here, the YUV 422 format is a format which has luminance (Y) information for every one pixel without reducing the same, and averages the chrominance (U, V) information is a format averaged in the horizontal two pixels. In this YUV 422 format, the reason why the chrominance (U, V) information of the horizontal two pixels is averaged is because of the visual characteristic hard to be judged as the image degradation for human eyes.
As a result, the input display data is 48 bits for every horizontal two pixels, while in the YUV 422 format, it is possible that the input display data is 32 bits for every horizontal two pixels. Thus, the data amount-stored in the memory can be relatively reduced by about 30 percent.
In relation to the YUV processing (YUV format conversion processing) for the display data, Japanese Patent Application Laid-Open No. 2003-123062 and Japanese Patent Application Laid-Open No. 2005-055824 can be cited.
In Japanese Patent Application Laid-Open No. 2003-123062, a plurality of different tables performing YUV-Y′U′V′ conversion processing are provided in an LCD control unit, and from among these conversion tables, one table is selected so as to perform the YUV-Y′U′V′ conversion. As a result, the YUV-Y′U′V′ conversion matching the display image is performed, and after that, the YUV-RGB conversion processing is performed.
In Japanese Patent Application Laid-Open No. 2005-055824, an RGB data is converted into a YUV data, and the converted YUV format is separated into a luminance component and a chrominance component, and by giving consideration to the characteristic of a color space of the input data, data compression processing is performed. At this time, the luminance (Y) data realizes a lossless compression by Huffman coding processing (that is, compression ratio is changed depending on data), and the chrominance (U, V) data realizes reduction of information amount of the display data by performing quantizing lossless compression after reducing the information amount.